The saxaul locust, Dericorys albidula Serville (Orthoptera: Dericorythidae) is a major pest of saxaul plants in Qom province of Iran. During 2005-2006, different nymphal instars of bands of D. albidula were treated by aerial spraying of Metarhizium anisopliae var. acridum (Green Muscle®). The gasoline formulation of M. anisopliae var. acridum isolate IMI 330189 was applied in different conidial concentrations (106, 107, 108, 109, 1010 and 1013 spores mL-1) that were prepared in sterile distilled gasoline. Results showed that various concentrations significantly affected the 2nd, 3rd, 4th and 5th nymphal instars of D. albidula compared to control. In addition, there were no differences in the effects of the different concentrations in 2005, but the differences were significant in 2006. Concentration 1010 killed 100% of tested insects 15 d after treatment. Comparing the results of the two years showed that the susceptibility of nymphs in the second year (2006) was higher than in the first year (2005). In conclusion, the results of this study indicated that the fungal insecticide M. anisopliae var. acridum, diluted in gasoline, was efficacious with the nymphal instars of locust D. albidula in 2005 and 2006.

The saxaul locust, Dericorys albidula Serville (Orthoptera: Dericorythidae), is a major pest of saxaul plants in Qom province in central Iran. To prevent shifting sands, the desert areas in Qom are covered by two species of saxaul plant, Haloxylon persicum Bunge ex Boiss. & Buhse (zard-tagh in Persian) and Haloxylon ammodendron (C.A. Mey.) Bunge (siah-tagh in Persian). The locust D. albidula is a monophagus pest and feeds on saxaul plants (Haloxylon spp.) and causes severe damage in different growth periods (Adeli and Abaei, 1989; Moniri, 1998). During the last two decades, most outbreaks of D. albidula have likely developed due to of the expansion of cultivated Haloxylon trees. To limit damage to crops, treatment is required virtually every year, usually with large amounts of broad-spectrum chemical pesticides that pollute the environment and present health and safety issues, as well as exacerbating locust problems due to the loss of natural enemies (Moniri et al., 2005).

In recent years, some studies have focused on developing fungal insecticides to control locusts and grasshoppers (Prior, 1992; Moore and Prior, 1993). Four to five hundred species of fungi have pathogenic effects on insects. There are entomopathogenic species among all five sub-divisions of fungi (Mastigomycotina, Zygomycotina, Ascomycotina, Basidiomycotina, and Deuteromycotina) (Burges and Hussey, 1971; Whitten and Oakeshott, 1991; Starnes et al., 1993). Fungi penetrate the insect cuticle. The host can be infected by direct treatment and by transportation of inoculums from treated insects to untreated ones (Lacey et al., 1999; Quesada-Moraga et al., 2004). After attaching itself to an insect host, the conidium penetrates the cuticle with the help of pressure of the germ tube and enzymatic degradation (Starnes et al., 1993). As well, the infection can occur through the respiratory system (Burges and Hussey, 1971). The genus Metarhizium (Deuteromycotina: Hyphomycetes) includes three species: M. anisopliae (Metschn.) Sorokin, M. flavoviride (W. Gams & Rozsypal) and M. album (Petch). Unlike M. anisopliae, which affects a large number of insect orders, the other two species have a more restricted host range (Rombach et al, 1986; 1987). One of the most promising biological agents for controlling locusts and grasshoppers is the Acridid-specific fungal pathogen Metarhizium anisopliae var. acridum (formerly Metarhizium flavoviride) (Ascomycota: Hypocreales) (Langewald et al., 1997; Driver et al., 2000; Hunter et al., 2001; Peng et al., 2008; Bischoff et al., 2009; USDA, 2010). Depending on insect species and size, M. anisopliae results in host death between 3 and 4 d after infection (Whitten and Oakeshott, 1991; Starnes et al., 1993). A variety of this species, M. anisopliae var. acridum, under the commercial name Green Muscle, has been developed to control locusts and grasshoppers (Thomas et al., 2000). Using oil-based formulations has improved the virulence of conidia and resulted in very promising acridid control (Symmon, 1992; Bateman, 1997).

According to the authors' knowledge, reports about D. albidula are limited and there have been no investigations regarding the effect of mycoinsecticide on the species. In this study, we investigated the effect of various concentrations of M. anisopliae on different nymphal stages of D. albidula under laboratory condition over two years.

MATERIALS AND METHODS

Insect rearingThe initial population of the locust D. albidula was collected from desert areas in Qom province, Iran. Saxaul bushes (H. ammodendron) were used for feeding and egglaying. Insect stock was maintained at 23 ± 2 °C and 35 ± 5% relative humidity (RH) under a 16:8 (L:D) cycle in a growth chamber.

Bioassay testsThe miscible flowable gasoline formulation of M. anisopliae var. acridum isolate IMI 330189 with 400 g (2 x 1013) viable conidia per liter was used in the experiments. A spray method was used in the toxicity assays. Different conidial (Green Muscle®) concentrations (106, 107, 108, 109, 1010, and 1013 spore mL-1) were prepared in sterile distilled gasoline. As a control, gasoline was used. Two-year-old saxaul bushes were cultured in plastic pots (15 cm in diameter and 25 cm height) and 10 1-d-old nymphs were placed on each bush. Ten milliliters of each concentration (fungus diluted in gasoline) and control group were sprayed on bushes. The mortality was recorded after 15 d from treatments. The experiments were repeated three times. This method was performed for various nymphal instars (2nd, 3rd, 4th, and 5th).

Insecticidal effect of M. anisopliae on D. albidula in 2005Figures 1-3 show the effect of various concentrations of M. anisopliae var. acridum on the 3rd (Figure 1) 4th (Figure 2) and 5th (Figure 3) nymphal instars of D. albidula in the first year (2005). Figure 1 reports that various concentrations (106, 107, 108, and 109) significantly affected the 3rd nymphal instars of D. albidula compared to control. In addition, the effect of concentrations was not different (df = 5, 12, F = 10.47, P = 0.0004). Concentrations 109 and 1010 had a significant effect on the 4th nymphal instars of D. albidula (Figure 2). There were no differences among these concentrations (df = 2, 6, F = 108, P < 0.0001). Comparison of toxicity of different concentrations of M. anisopliae var. acridum on the 5thnymphal instars of D. albidula indicated that nymph mortality rates at all concentrations (106, 107, 108, 109, and 1010) were significantly higher than for the control (Figure 3). The toxicity levels of the various concentrations were different but not significant (df = 5, 12, F = 15.54, P < 0.0001).

In this study, the effect of various concentrations of conidia of M. anisopliae var. acridum on different nymphal instars of D. albidula was assayed under laboratory conditions over two years. This was the first study of the efficacy of a fungal insecticide on D. albidula. The results indicate that M. anisopliae var. acridum affects the 2nd, 3rd, 4th, and 5th nymphal instars of this important pest of saxaul plants. In our study, the susceptibility of D. albidula nymphs was assessed over 2 yr using similar concentrations. The susceptibility of nymphs was greater in the second year (2006) than in the first year (2005). Unlike our study, Peng et al. (2008) researched the effect of M. anisopliae var. acridum against oriental migratory locusts, Locusta migratoria manilensis (Meyen) over two years (2002 and 2003) and reported that mortality rates caused by this fungus on the aforementioned locust was relatively similar in both years. In this study, the conidia of M. anisopliae were diluted in gasoline and results of this method were evaluated well. Using petroleum products had been tested previously. de Faria et al. (2002) showed that conidia of M. anisopliae var. acridum in a mixture of soybean oil and kerosene had a good effect on the locust, Rhammatocerus schistocercoides Rehn. Because saxaul plants are not used for food, using petroleum products can be a suitable method of control. In the current study, mortality was recorded after 15 d. Similar to our results Lomer et al. (1997) stated that M. anisopliae killed over 90% of nymphs after 15 d. These similar results indicate that 15 d is an adequate period for the fungus to take effect. Lomer et al. found that a dose 1010 of M. anisopliae var. acridum killed 100% of second instar nymphs of D. albidula. Magalhaes et al. (2000) stated that a 2 x 1013 concentration of M. anisopliae var. acridum caused 88% mortality on the 2nd nymphal instar of R. schistocercoides. As well, Alves et al. (1999) observed that M. anisopliae caused 79-90% mortality of short-horned locust in Africa, Brazil, and Australia. Kassa et al. (2004) examined the effect of Green muscle on Locusta migratoria (R. & F.) and reported that this compound can be effective on this pest.

CONCLUSION

In conclusion, the results of this study indicate that fungal insecticide M. anisopliae var. acridum diluted in gasoline was highly effective in causing mortality of 2nd, 3rd, 4th, and 5th nymphal instars of locust D. albidula in 2005 and 2006. As well, the current report shows that mortality of all stages in the second year (2006) was higher than in the first (2005).

ACKNOWLEDGEMENTS

We thank the Agricultural and Natural Resources Research Center of Qom, Iran, for its assistance.

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